Natural gas commonly serves as a fuel for power generation or a fuel for domestic use. The process of obtaining natural gas from an earth formation typically includes drilling a well. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas may necessarily need to be transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the relatively large volume occupied by gaseous natural gas. Therefore, the process of transporting natural gas may include chilling and/or pressurizing the natural gas in order to liquefy it. However, the expenditures associated with liquefaction may be high and liquefaction may not be economical for formations containing small amounts of natural gas.
Formations that include small amounts of natural gas may be small natural gas fields or natural gas as a byproduct of oil production (“associated gas”). In the past, associated gas may have been flared. However, current environmental concerns and regulations may discourage or prohibit this practice.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline, jet fuel, kerosene, and diesel have been decreasing and supplies may not meet demand in the coming years. Fuels that are liquid under standard atmospheric conditions have the advantage that they can be transported more easily in a pipeline than natural gas, since they do not require the energy, equipment, and expense required for liquefaction.
Thus, for all of the above-described reasons, there may be interest in developing technologies for converting natural gas to more readily transportable liquid fuels. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted to form a mixture of CO and H2 (“synthesis gas” or “syngas”). This syngas generation usually occurs either by dry reforming, steam reforming, or partial oxidation, respective examples of which are set forth below for methane:CH4+CO2→2CO+2H2  (1)CH4+H2O→CO+3H2  (2)CH4+½O2→CO+2H2  (3)Reactions (1) and (2) are endothermic and reaction (3) is exothermic. Examples of syngas processes are disclosed in Gunardson, “Industrial Gases in Petrochemical Processing,” 41–80 (Marcel Dekker Inc. 1998), and in U.S. Pat. No. 6,402,989 to Gaffney, and U.S. patent application Ser. No. 20020115730 to Allison et al., all incorporated herein by reference.
While its use is currently limited as an industrial process, the direct partial oxidation or catalytic partial oxidation of methane has recently attracted much attention due to its inherent advantages, such as the fact that due to the heat that is released during the process, there is no requirement for the continuous input of heat in order to maintain the reaction.
In the second transformation, known as hydrocarbon synthesis (e.g., Fischer-Tropsch or FT synthesis), carbon monoxide reacts with hydrogen to form organic molecules containing carbon and hydrogen. An example of a Fischer-Tropsch process is disclosed in U.S. Pat. No. 6,333,294 to Chao et al., incorporated herein by reference.
Another example of a Fischer-Tropsch reaction scheme comprises charging a feed gas comprising hydrogen and carbon monoxide (e.g., syngas) to an FT reactor. H2/CO mixtures suitable as a feedstock for conversion to hydrocarbons according to the process of this invention can be obtained from light hydrocarbons such as methane by means of steam reforming, partial oxidation, or other processes known in the art or disclosed herein. The hydrogen may provided by free hydrogen, although some Fischer-Tropsch catalysts have sufficient water gas shift activity to convert some water to hydrogen for use in the Fischer-Tropsch process. The molar ratio of hydrogen to carbon monoxide in the feed may be greater than 0.5:1 (e.g., from about 0.67 to about 2.5). If cobalt, nickel, and/or ruthenium catalysts are used, the feed gas stream may contain hydrogen and carbon monoxide in a molar ratio of about 1.6:1 to about 2.3:1. If iron catalysts are used, the feed gas stream may contain hydrogen and carbon monoxide in a molar ratio between about 1.4:1 and about 2.3:1. The feed gas may also contain carbon dioxide and/or a low concentration of compounds or elements that have a deleterious effect on the catalyst, such as poisons. In some instances, the feed gas may need to be pretreated to ensure that it contains low concentrations of sulfur or nitrogen compounds such as hydrogen sulfide, hydrogen cyanide, ammonia and carbonyl sulfides.
The feed gas is contacted with the catalyst in a reaction zone. Mechanical arrangements of conventional design may be employed as the reaction zone including, for example, fixed bed, fluidized bed, slurry bubble column or ebullating bed reactors, among others. Accordingly, the size and physical form of the catalyst particles may vary depending on the reactor in which they are to be used.
The Fischer-Tropsch process may be run in a continuous mode. In this mode, the gas hourly space velocity through the reaction zone typically may range from about 50 to about 10,000 hr−1, or from about 300 hr−1 to about 2,000 hr−1. The gas hourly space velocity is defined as the volume of reactants per time per reaction zone volume. The volume of reactant gases is at standard conditions of pressure (1 atm or 101 kPa) and temperature (0° C. or 273.16 K). The reaction zone volume is defined by the portion of the reaction vessel volume where reaction takes place and which is occupied by a gaseous phase comprising reactants, products and/or inerts; a liquid phase comprising liquid/wax products and/or other liquids; and a solid phase comprising catalyst. The reaction zone temperature may be in the range from about 160° C. to about 300° C. The reaction zone may operate at conversion promoting conditions at temperatures from about 190° C. to about 260° C. The reaction zone pressure may be in the range of about 80 psia (552 kPa) to about 1000 psia (6895 kPa), from 80 psia (552 kPa) to about 600 psia (4137 kPa), or from about 140 psia (965 kPa) to about 500 psia (3447 kPa).
A Fischer-Tropsch product stream may contain hydrocarbons having a range of numbers of carbon atoms, and thus having a range of weights. Thus, the product produced by conversion of natural gas commonly contains a range of hydrocarbons including light gases, gases, light naphtha, naphtha, kerosene, diesel, heavy diesel, heavy oils, waxes, and heavy waxes. These cuts are approximate and there may be some degree of overlapping of components in each range.
The heat generation of the partial oxidation reaction can be a double-edged sword. Temperatures at typical reaction conditions can be in excess of 1,000° C. The high temperatures can lead to problems such as carburisation and/or metal dusting. Metal dusting may be a form of corrosion of metals that occurs in high-carbon activity, low-oxygen potential carburizing gaseous environments, even at intermediate temperatures of 350° C. to 900° C. Metal dusting may be characterized by the production of a powdery mixture of carbon, metal particles and sometimes oxides and carbides or voluminous coke deposition. These corrosion products (dust) can be eroded by a gas flow leaving pits, grooves, or holes on the metal surface. This phenomenon causes metal disintegrated which leads to early pit penetration and failure of plant components. In addition, the coke deposits can reduce syngas cooling efficiency by reducing the gas velocity (e.g., hinder gas flow) and reduce the thermal conductivity because the coke is an insulator. As a result, the residence time of syngas at elevated temperature which is within the metal dusting range may be increased dramatically. Thus, there is a desire to decrease carburisation and/or metal dusting that occurs in the components of a syngas reactor.
The CO2 and H2O concentrations in partial oxidation reactors are preferably low, which places catalytic partial oxidation reactors in the most severe regions of the metal dusting spectrums (as is illustrated in FIG. 2). It has not previously been known whether nickel based alloy or other metals components can help decrease the effects of metal dusting in relatively severe catalytic partial oxidation reactors.
Therefore, it may be desirable to find materials that can withstand the harsh conditions of temperature and pressure inside a partial oxidation reactor. It may also be desirable to find out what metallurgy can be used in order to reduce or prevent carburisation and/or metal dusting effect during catalytic partial oxidation of light hydrocarbons.